Abu Hurayrah رضي الله عنه reported that the Messenger of Allah صلى الله عليه وسلم , said, “Allah, the Mighty and Exalted said: ‘Every act of the son of Adam is his except for fasting; for indeed it is for Me and I will give the reward of it.’ Fasting is a salvation from the Hell-Fire. Therefore if one of you is fasting he should not be obscene in his language nor indulge in unnecessary argument. If someone insults him or tries to pick a fight with him, he should say to that individual: “I am fasting.””

“I swear by him whose hand is Muhammad’s soul, the bad breath from the mouth of the individual who fasts is, with Allah, better than the fragrance of the musk. The one who fasts has two occasions of happiness, the first one is his happiness when he breaks his fast, and the second happiness is when he receives his reward with his Lord” [collected by Al-Bukhari and Muslim]

“Every act of the son of Adam is His, every good deed will be multiplied for him ten to seven hundred times, except fasting, for verily it is Mine and I give the reward for it. The slave restrains his sexual desire(s) and food for My sake”

Shaykh Uthaymeen رحمه الله mentions;
This great Hadith indicates the virtues of fasting from many aspects;

The Fourth Aspect: The bad breath from the one who fasts is better than the smell of musk with Allah, because it is from the traces of fasting. Due to that it is beloved to Allah more than the smell of musk. If this indicates anything, it indicates the magnitude of fasting with Allah. Even the thing that is disliked by the people and is disgusting to them, with Allah it is beloved because it derives from the (slaves) obedience to Allah by the way of fasting.